The present invention relates to nuclear power plants and service assist apparatus for motor pumps and especially to apparatus that is useful in facilitating the servicing of inverted main coolant pump motors for which there is limited access space and limited access time in a nuclear power plant.
In typical prior nuclear power plant applications, conventional shaft seal type system have been used or where canned coolant pumps have been used they have been mounted right-side up, i.e. with the motor end of the unit facing upwardly. Even through the motor and its casing are very heavy (such as 57,000 pounds), the motor in such a canned pump unit can be removed for servicing or replacement in a straightforward manner because vertical access space is normally available for use of conventional disassembly tools and lifting equipment in removing the casing studs and lifting the motor upwardly from the pump unit. In fossil power plants where inverted canned motor pumps have been used for some time, vertical access space for pump motor removal has been relatively unrestricted so that conventional plant service equipment and tools can be used by service personnel.
In a new nuclear power plant design developed by the assignee of the present application in which safe and reliable operation is based on passive control, each vertically elongated steam generator is contained within a relatively narrow vertical chamber with main canned coolant pumps located in an inverted position with the pump casing suction nozzle welded to the bottom of the steam generator channel head for operation in the reactor coolant looping. The bottom of the pump unit is relatively close to the chamber floor so that vertical access space is highly limited. In addition, access time is effectively limited by considerations of radiation exposure to service personnel.
The nuclear power plant operating and safety advantages for the inverted canned motor pump configuration are significant. First, auxiliary fluid systems needed to support a canned motor pump are much less complex than those needed for a shaft seal type pump. Further, the canned motor pump inherently reduces small (LOCA) coolant loss potential, has a demonstrated record of high reliability and is more tolerant of off design conditions than the shaft seal pump is. The close coupling of the pump suction to the bottom of the steam generator channel head eliminates the need for a crossover coolant piping leg thereby improving the small break (LOCA) transient. Such close coupling also reduces loop pressure drop, simplifies the foundation and support system for the steam generator, pumps and piping, and considerably reduces the cost and complexity of primary coolant loop piping.
With the described inverted disposition, the pump units are located such that removal of the pump motor requires that the motor be supported while the casing studs are withdrawn. Once the studs are removed, the motor must be dropped downwardly to clear the pump unit flange and it then must be transferred in the lateral direction to clear the steam generator. The pump motor must be moved to a point where a narrow shaft like vertical space is provided within the chamber for vertical lifting of the motor upwardly along the steam generator to a point where it can be removed from the chamber. Installation of a motor pump involves reverse requirements.
Accordingly, it is desirable that apparatus be developed to facilitate the servicing of inverted canned motor pumps under the described conditions.